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Synopsis
Water is the most important substance on the planet. Its unique and versatile hydrogen bond network underlies many of the processes responsible for life as we know it. Yet, and despite centuries of study, vital questions regarding the intrinsic nature of water remain unanswered. One route to this knowledge is via the development of a "universal first principles" water model. Such models are currently under development via collaborations between our group and theoreticians, based on high precision terahertz spectra and detailed calculations of water clusters. The bend and stretching vibrations of the hydrogen bonds, with the associated quantum tunneling motions, provide a very sensitive measure of the water potential surface.
250. Keutsch, F. N., and Saykally, R. J., “Water Clusters: Untangling the mysteries of the liquid, one molecule at a time”, PNAS 98, 10533-10540 (2001).
Recent work in the group has aimed to implement two new types of laser systems to probe previously inaccessible regions near 300 and 500 cm-1. Solid-state Schottky diode frequency multipliers currently produce output powers on the order of 10 μW over a broad range of frequencies (i.e. 150 GHz tunability for a given multiplier array). In ongoing collaborations with Prof. Peter Siegel (CalTech/JPL), we have performed spectroscopy studies of water clusters using advanced THz source technology suitable for this purpose. We have also established collaborations with Dr. Alan Lee and his company LongWave Photonics, using arrays of quantum cascade lasers (QCLs). These devices, while well-established in the mid-IR, are still experimental in the THz region, but provide much higher power (tens of milliwatts) with continuous tunability and operate at higher frequencies. These two technologies are thus complementary.
395. Lin, W., Steyert, D. W., Hlavacek, N. C., Mukhopadhyay, A., Page, R. H., Siegel, P. H., Saykally, R. J. "Terahertz Vibration-Rotation-Tunneling Spectroscopy of the Propane-Water Dimer: the ortho--state of a 20 cm-1 Torsion" Chem. Phys. Lett., 614, 167-171 (2014).
405. Cole, W. T. S., Hlavacek, N. C., Lee, A. W. M., Kao, T.-Y., Hu, Q., Reno, J. L., Saykally, R. J., "A Terahertz VRT Spectrometer Employing Quantum Cascade Lasers" Chem. Phys. Lett., 638, 144-148 (2015).
The group has also been working through old unassigned data using new pattern recognition algorithms to identify transitions in the region near 500 cm-1. Thus far, spectra of the water dimer, pentamer, and hexamer have been identified and analyzed. Surprisingly, the transitions in this region show a dramatic enhancement (100-10000x) of cluster tunneling motions related to hydrogen bond breaking/formation. The implication of this enhancement is the subject of ongoing studies.
406. Cole, W. T. S., Fellers, R. S., Viant, M. R., Leforestier, C., Saykally, R. J. "Far-Infrared VRT Spectroscopy of the Water Dimer: Characterization of the 20 μm Out-of Plane Librational Vibration" J. Chem. Phys., 143, 154306 (2015).
414. Cole, W. T. S., Fellers, R. S., Viant, M. R., Saykally, R. J. "Hydrogen bond breaking dynamics in the water pentamer: Terahertz VRT spectroscopy of a 20 μm libration" J. Chem. Phys., 7, 014306 (2017) (SI).
418. Cole, W. T. S., Saykally, R. J. "Hydrogen bond network rearrangement dynamics in water clusters: Effects of intermolecular vibrational excitation on tunneling rates " J. Chem. Phys., 147, 064301 (2017) (SI).
There are two directions to our current water cluster spectroscopy experiments:
1) Extending measurements to larger clusters, seeking to refine descriptions of hydrogen bond cooperativity
411. Cole, W. T. S., Farrell, J. D., Wales, D. J., Saykally, R. J. "Structure and torsional dynamics of the water octamer from THz laser spectroscopy near 215 μm" Science, 352, 1194-1197 (2016) (SI).
2) Measuring key hydrogen bond vibrations that have not previously been accessible, e.g. out of plane librations in the 300 and 500 cm-1 region.
406. Cole, W. T. S., Fellers, R. S., Viant, M. R., Leforestier, C., Saykally, R. J. "Far-Infrared VRT Spectroscopy of the Water Dimer: Characterization of the 20 μm Out-of Plane Librational Vibration" J. Chem. Phys., 143, 154306 (2015).
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